A mobile communication system typified by a cellular phone has been increased in penetration on a global scale. Several communication systems have been used according to the locality. As one of important techniques used in the mobile communication system, there is known a technique called APC (Auto Power Control) which controls transmitted power of a terminal according to received power at a terminal and a base station. A first reason why APC is required, resides in a reduction in power consumption. When the terminal is located near the base station, the transmitted power of the terminal is lowered to make it possible to achieve a reduction in power consumption of the terminal as compared with the case in which transmitted power is constant. A second reason is a near-far problem at a CDMA (Code Division Multiple Access) system.
In the CDMA, a plurality of terminals perform spread spectrum modulation on signals according to a spread spectrum using codes different from one another and transmit them. Thus, the same frequency band can be shared among the plurality of terminals. When, for example, a terminal B that exists at a distance far away from a base station A and a terminal C that exists at a distance near from the base station A are considered, a signal (desired wave) from the terminal B is subjected to level interference due to a signal (undesired wave) of the terminal C, thus resulting in the difficulty of accurate reception. In order to solve this problem, it is essential to precisely control power transmitted by each terminal and keep received power at the base station constant.
As an example of a power control circuit for realizing APC, an example that detects part of output power of a transmitted power amplifier and controls the transmitted power amplifier in such a manner that the result of detection reaches an ordered power level from a base station has been disclosed in a patent document 1: Japanese Patent Laid-Open No. H5 (1993)-291854 (Page 3-4, FIG. 1). As another example, an example that detects part of output power of a power amplifier, and controls an available attenuator for changing an input level of the power amplifier to thereby sample and hold a control voltage of the available attenuator when the result of detection reaches a desired value has been disclosed in a patent document 2: Japanese Patent Laid-Open No. H8 (1996)-274560 (Page 5-6, FIG. 1). As a further example, an example has been disclosed in a patent document 3: Japanese Patent Laid-Open No. 2004-7443 (Page 11, FIG. 1), wherein in a wireless communication equipment having a phase control loop for controlling the phase of a carrier of a transmitted signal and an amplitude control loop for controlling output amplitude of a power amplifier circuit, an available gain amplifier circuit is disposed at the midpoint of a feedback circuit of the amplitude control loop, and the gain of the same amplifier circuit is controlled by a digital circuit to thereby control the output power of the power amplifier. As a still further example, an example has been disclosed in a patent document 4: Japanese Patent Laid-Open No. 2000-228646 (Page 4-5, FIG. 1), wherein two available gain devices are disposed ahead of a power amplifier section, and the result of detection of power outputted from the power amplifier section results in a value corresponding to control from a base station and the two available gain devices are controlled such that a change in phase does not occur. As a still further example, an example that detects part of output power of a transmitted power amplifier and controls the transmitted power amplifier by a digital circuit such that the result of detection complies with instructions given from a base station has been disclosed in a patent document 5: Japanese Patent Laid-Open No. H8 (1996)-274559 (Page 3, FIG. 3).